This invention relates to a solar thermal electric power system. The solar thermal electric system is provided with a heat storage device because of intermittent supply of solar energy from the sun. Various kinds of heat storage devices are known, and there have been proposed many solar thermal electric power systems, some of which will be described hereinafter.
A first example of a prior art solar thermal electric system is disclosed in the Japanese Laid-open Patent Application No. Sho 50-154852, in which a solar collector system is connected via a heat storage vessel to a power generating system. The solar collector system has a first passage including a solar collector, a heat storage device and a pump to provide a closed circulation loop. The power generating system has a second passage including a heat storage vessel, a turbine, a condenser, and a pump to provide a another closed circulation loop. The generator is connected to the turbine. A thermal medium in the first passage is heated by solar energy which has been collected by the solar collector, pressurized by the pump, and then fed to the heat storage vessel. The heat storage vessel is filled with a thermal storage material. The heat of thermal medium flowing through the first passage is conducted to the thermal storage material in the heat storage vessel. Then, the temperature of the thermal medium is lowered and the thermal medium is delivered back to the solar collector. Water is circulated through the second passage by means of a pump provided in the second passage. Water flowing through the second passage is heated and vaporized into steam while passing through the heat storage vessel. The steam thus produced is delivered to the turbine for driving the same. Steam is condensed into the form of water, and then delivered to the heat storage vessel.
However, this prior art system suffers from the following disadvantages. In order for the heat of thermal medium flowing through the first passage to be transferred to the thermal storage material, a temperature difference or thermal head should be at least 50.degree. C. to 100.degree. C. On the other hand, when steam is to be produced from the thermal storage material for driving a turbine, a temperature difference as above is required. A temperature difference of at least 200.degree. C. is required between the steam from the solar collector, and the steam at the entrance of a turbine. In this regard, the efficiency of a turbine plant is increased, as the temperature of steam at the entrance of a turbine is increased, so that a decrease in temperature of steam at the entrance of a turbine due to the use of the heat storage vessel leads to a decrease in efficiency of a turbine plant and associated heat generating plant.
In addition, the above-described prior art system adopts a counterflow type in an attempt to increase a temperature efficiency of heat exchange between the thermal medium and a turbine drive medium (a fluid for driving a turbine, for instance, steam). However, since an outlet for a turbine driving medium and an inlet for the thermal medium are positioned close to each other, a temperature change in the thermal medium directly affects the temperature of a turbine driving medium, thereby causing a thermal shock on a turbine. Furthermore, when there is a temperature drop of steam at an outlet of the solar collector due to a sharp drop in amount of solar radiation, then the heat in a high temperature portion of a heat storage vessel in the vicinity of an outlet for steam, of the heat storage vessel is shifted to a low temperature portion of the heat storage vessel, thus failing to obtain high temperature steam for use in driving a turbine.
A second prior art system is disclosed in Pages 184 to 190, particularly in FIG. 13, 2-3, Colorado State University et al: "Solar Thermal Electric Power System" ANNUAL PROGRESS REPORT (National Technical Information Service), Report No.: NSF/RANN/SE/GI-37815/PR/73/4 (January 1974). The detailed description of a heat storage vessel is given in pages 138 to 143, and page 202. According to the system given in FIG. 13, 2-3, there is provided a heat storage vessel filled with water internally, i.e., a steam accumulator. Part of the steam produced by a solar collector is introduced to an accumulator for heating water therein. Heat is stored in the accumulator in the form of a saturated water and saturated steam, and then the saturated water is self-vaporized and then the saturated steam is supplied to a turbine.
However, this type of prior art system also suffers from another disadvantage in that since the accumulator is provided in the form of a pressure vessel of a large mass, thermal storage under a high pressure is impossible. In this respect, the pressure in the accumulator should be limited to 70 atg which is customary for a nuclear power pressure vessel, according to the state of the art. The maximum temperature of steam in this case is 280.degree. C., and the steam temperature on this order can hardly bring about a high efficiency for a turbine plant. Furthermore, the production of steam lowers a pressure in the accumulator, and hence the temperature of steam to be supplied to a turbine is lowered, as the time goes on, so that energy collected at a high temperature can not be effectively used without a temperature loss.
Another example of a prior art system is given in pages 166 to 173; "Dynamix Conversion of Solar Generated Heat to Electricity "NASA CR134724" (August 1974) by J. C. Powell, et al.
A third prior art system is given on line 22, pages 138 to line 6, page 139, "Solar Thermal Electric Power System", which has been cited earlier. This system includes a heat storage vessel filled with a thermal storage material for effecting the phase change between solid and liquid. A heat storage vessel filled with a thermal storage material (for instance molten salt) effecting the aforesaid phase change of solid-liquid is connected to a pipe route between a collector field and a turbine. Two or more heating pipes, through which steam flows from the collector field, are placed in the heat storage vessel, with thermal storage material encompassing the same.
As compared with the other described prior art systems, this prior art system permits the maintenance of a steam temperature for a turbine at a high level, but suffers from a drawback in that heat is stored in the form of a superheated steam, and thus a long period of time is required for storing heat. For instance, in case the temperature of superheated steam is dropped by 50.degree. C. (temperature of steam to be supplied to a turbine is 400.degree. C.) for thermal storage, then an enthalpy head of steam will be about 25 kcal/kg. On the other hand, when the duration of sunshine is shortened, as a result, a solar collector can no longer be used, the enthalpy of about 650 kcal/kg is required for obtaining superheated steam on the order of 400.degree. C. in a water feeding condition (in the case of the use of a deaerator, about 140.degree. C. steam) from a heat storage vessel. In case steam provides the same flow rate upon heat storage and heat radiation, the time required for thermal storage amounts to about 26 times as long as the time required for heat radiation.